text:
Do the trim wheels on the Boeing 737NG move by themselves during the takeoff roll?
After the stab trim is set before take off, will it move by itself during the take off roll? If yes, why? Where can I find the reference?

The stab trim should not move by itself during the takeoff roll on the 737 NG.
The stabilizer can in principle be moved by the following things:

the stab trim switches on the control wheels: these are available during the takeoff roll, but do not operate automatically
the autopilot trim: the autopilot is turned off during the takeoff roll, so no trim inputs are made here
stabilizer trim wheels: these are mechanically linked and allow manual adjustment of the stabilizer, but no automatic operation
the Speed Trim System (STS): the STS starts to be active 10s after takeoff, so not during the takeoff roll

The details are described in the Flight Crew Operating Manual (FCOM):

Stabilizer
The horizontal stabilizer is positioned by a single electric trim motor controlled
through either the stab trim switches on the control wheel or autopilot trim. The
stabilizer may also be positioned by manually rotating the stabilizer trim wheel.
Stabilizer Trim
Stabilizer trim switches on each control wheel actuate the electric trim motor
through the main electric stabilizer trim circuit when the airplane is flown
manually. With the autopilot engaged, stabilizer trim is accomplished through the
autopilot stabilizer trim circuit. [...]
Speed Trim System
The speed trim system (STS) is a speed stability augmentation system designed to
improve flight characteristics during operations with a low gross weight, aft center
of gravity and high thrust when the autopilot is not engaged. [...]
Conditions for speed trim operation are listed below:

Airspeed between 100 KIAS and Mach 0.5
10 seconds after takeoff
5 seconds following release of trim switches
Autopilot not engaged
Sensing of trim requirement

(Boeing 737NG FCOMv2 - 9.20.8 - Flight Controls, System Description)
Related:

When was the Speed Trim System implemented on the Boeing 737?
Was the speed trim system on the B737 Classic modified to counter increased thrust?


text:
What is the EGT probe output tolerance?
What is the 'average' or general tolerance (+/-) for EGT probes output on a 'typical' commercial turbofan engine?

It is much more complex than you can imagine. I found a master thesis entitled Investigation of Sensor Faults in a Gas Turbine of Politecnico di Torino, studying the CFM56 turbofan.
It states that thermocouples used in engines have an accuracy of 0.25% of the full scale. In the particular case of the thesis, they analyzed two different type of thermocouples:

This thermocouple type presents an accuracy of ± 2.2°C or 0.75% for category 2, while accuracy of ± 1°C or 0.4% for category 1.

However, this is just the sensor itself. The entire thesis discussed about inaccuracies when considering the whole chain (position, etc.) that may be also > 10 °C (see Table 4.1).

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What is that green rectangle next to altitude readout on Boeing aircraft?

Does anyone know what this green rectangle represents? It's from a Boeing 777 PFD. I can't find answers anywhere.

It indicates that the altitude is below 10,000 ft.
My Boeing 777 FCOM shows the same rectangle in the PFD description, but does not mention any details about it.
However, the Boeing 747 FCOM includes more details:

6 Ten Thousand Digit Display
Cross hatch displays when altitude is below 10,000 feet.

(Boeing 747-400 FCOM 10.10.28 - Flight Instruments, Displays - Controls and Indicators)

@J... I think it's more that 10,000ft is a magic number for "are we higher or lower than those mountaintops" reasons

@JackDeeth I very, very much doubt that... there are hundreds of mountains above 10,000' all over the world (and similarly, many huge regions where far lower altitudes are safe). The idea of 10,000' being any kind of safe-altitude rule of thumb is ridiculous.

@J... and there also being space on the display with the 1st digit being a 0 / not present!

@Tim I guess that is the primary function - to avoid ambiguity.  A black space might allow 3200 to be confused with 32000, and 03200 is just hard to read.

This is a crosshatch marking to show the altitude is below 10,000 feet.  See What is the meaning of the crosshatch marking on altimeters?

Any idea why it's green rather than white?

@JackDeeth My guess is because a digit in that position (above 10,000') would be white, so the green crosshatch reduces the chance of it being mistaken for a digit.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why are spoiler panels 5 and 10 locked in high speed flight on the B777?
From the Boeing 777 FCOM:

Spoiler panels are used as speedbrakes to increase drag and reduce
lift, both in flight and on the ground. The spoilers also supplement
roll control in response to control wheel commands. Spoiler panels 5
and 10 are locked out during cruise, depending on altitude and
airspeed.

What is the reason for locking out this particular spoiler pair?

Related: Why does one of the Boeing 777's spoilers deploy differently from the rest?


text:
Was there an incident when Airbus autopilot wasn't letting the airplane land?
I'm trying to track down a story about an incident involving an Airbus airplane my father told me about 25 years ago, when he was working at a Boeing supplier.
Reportedly, what happened was the airplane's autopilot was set to bring them in to a certain airport, and the pilots noticed that it was descending too early.  So, they took over the controls, lined up manually, and at about a hundred feet up, the autopilot wouldn't let them descend further.  So, they did a go-around, tried the approach again, and the same thing happened.  At this point, they diverted to another airport and landed without incident.  Turned out that the airplane's database had an incorrect position for their destination: the plane thought they were overshooting the runway, and wouldn't let them perform what it saw was an off-airport landing.
Was this based on an actual incident, or just one of the "Airbus airplanes don't let the pilot fly" stories circulating among Airbus competitors?

never heard of such accident nor such capability of the airbus autopilot. I put my chips on the second hypothesis.

If you find a notable reference for this story, it being retold on a website or in a book, then its a good fit for http://skeptics.stackexchange.com

I'd say this must be an urban legend. Not sure if it was like this everywhere, but 25 years ago, there existed a rule for safety critical systems, that any kind of automation was only allowed if it could be shut down completely by a human. Note that it's not more like this nowadays.

@Zane, not anymore? got any links/documents to support the claim?

I don't know about not letting the aircraft land, but there was an incident where the flight control system wanted to make sure they landed.

@voretaq7: No, it didn't. The aircraft didn't start to climb because it didn't have sufficient speed for it. The computer did prevent pulling up further, but if it didn't the aircraft would have stalled with the same overall result. They were simply too late applying power (which the computer would have done for them much earlier if they didn't disable it!!) and nothing could have saved them at that moment.

@Federico: Might in fact be that this is still a requirement in avionics, I don't know. It seems it's not a requirement for automotive, where software support emerged much later.

@Zane I find that extremely hard to believe, such requirement would create only problems and no benefits.

@Federico: which requirement? That a human can shut down the automation? I'd say this is a very natural requirement that causes no problem at all.

@Zane, that it got removed as you say in any kind of automation was only allowed if it could be shut down completely by a human. **Note that it's not more like this nowadays**

@Federico: I remember this from the safety critical systems lecture. I agree that this is dangerous. However, I think this is really true nowadays for automotive, maybe because it's less dangerous there. Might very well be that this is still a requirement in avionics and power plants.

@Zane we were speaking about aricraft here, not automotive. And you claimed that such a requirement was no more in place, that's why I asked for sources. Also, the subject had been address here before

@Zane: I don't think there is, or ever was, any such requirement for the kind of automation we are talking about here, i.e. something that translates direct human commands via non-trivial controller. In Airbus you sure can disable the FBW, but you have to use the circuit breakers and you would be left with elevator trim and rudder only (yes, the aircraft was demonstrated controllable with just that).

@Fedrico: sorry for being imprecise and puzzling you. No I don't have a source for avionics.

The details don't match up, but you may be interested in Air France Flight 296.

There is no such requirement that pilots must be able to override any automation in an aircraft. Simple example that this is the is FADEC systems for engine control. The FADEC computers know more about the engine at any given moment than any pilot (or two) can process in their heads. Gone are the days of having to monitor temps to make sure you don't burn up your very expensive turbofan engine. And yes, FADEC is avionics -- aviation electronics.

I believe this is a hoax1. I've read through several documents describing the protections provided by Airbus flight laws (this is not autopilot; autopilot is a separate layer on top of it) and have never seen any mention of any kind of ground proximity protection. Only standard (E)GPWS which yells "terrain, pull up".
Update: Of course now there is EGPWS that gets aircraft position from GPS and can yell "terrain, pull up" when there is no airport ahead, but 25 years ago there was only GPWS based on radar altimeter which didn't know anything about position and would switch to simply read out2 the radar altimeter value whenever gear (and maybe also flaps; not sure here) was extended.

1Might have been even attempt at FUD against the then novel technology.
2That is say "one thousand", "five hundred", "two hundred", "one hundred", "fifty", "forty", "thirty", "twenty", "retard"3 (and "ten" or "five" when it stops saying "retard" and the wheels are not on the ground yet). This is useful during approach so the pilot can concentrate less on the instruments and more on outside reference and in the final phase to help judge when flare should be initiated. It will also warn "sink rate" when the sink rate is excessive (more than -1000 ft/min, the stabilized approach criteria).
3At 20 or so feet, if the thrust levers are not in idle gate, the Airbus system starts announcing "retard" and keeps announcing it until the thrust levers are placed in the idle gate (or reverse). This is specific to Airbus. I believe it can also announce "minimums" when approach minima are configured in the flight management system.

I would categorise this as an urban legend, as there is no this kind of function or protection in Airbuses. Any protection will not kick in until you are trying to exit aerodynamical envelope of the aircraft.
But many of these stories are based on true events, as early Airbus pilots were mostly experienced second-generation-jet pilots, who just couldn’t wrap their head around with new technologies A320 offered. As the pilots could not understand what the flight management system was doing, they disconnected it with various level of success and told the stories as they perceived it.
It took decades for the industry to actually understand the complex interaction between the pilot and the machine and to develop levels of training required to operate these modern aircraft. Paradoxically, now that pilots understand better all the modes of the system, they have less hesitation to take over manually when required.

This urban legend has been going around for ages and one flight (not sure if it was the start or if there were others) which started the myth that Airbus pilots are just along for the ride was the LH2904 accident in WAW.
In this case, the spoilers and reversors did not deploy as the conditions for deploying were not met resulting in aircraft going off the runway at highish speed.
The cause of the accident was pilot error with contributory factor of aircraft system/lack of guidance in the AOM.
link to accident report here

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How can I calculate QNH from QFE?
Can someone please explain in simple terms how to work out your QNH from QFE?
For example, you take off at 0' but leave the circuit, what do you set the altimeter to? How do you know the QNH?

If you have it set to QFE, turn the knob to add the field elevation to the current altitude. Or set it to the field elevation on the ground. (Or get the QNH from a nearby ATIS.) :)

I found this googling helpful, especially this rule of thumb: "Divide airport altitude (in feet) by 30 feet. Add resultant number of millibars onto the QFE that you are given. Then you have QNH.". But please (!) wait for more profound answers, this was just what I found at Google.

Appreciate your help guys, i've googled and confused myself silly.

May I ask where do you fly that you use the terms QNH and QFE?

@user6035379 I think it's pretty much anywhere outside north america. Though QFE is very rarely used (I think the UK uses it more than others, it's also fairly common with gliders)

@user6035379 Very common in the UK when operating within the airfield zone.  It makes flying circuit height etc very easy. 1000 on the altimeter, job done.  We switch to QNH once we transition to en-route or otherwise leave the zone.

Respected Sir , since long time I was confused and unable to get satisfactory answer about QNH & QFE but at present I got completely satisfactory action .Dear Sir I highly recommend and appreciated your efforts. Thank you very much From Mr Ghulam Jilani ATC KSA

In metric:

1 milibar is approximately equal to 30 feet.  This calculation is therefore also approximate but is good for airfield elevations to several hundred feet since we round to the nearest millibar.
Divide the airfield altitude in feet by 30 to get the number of millibars above MSL. Add this to the QFE to get QNH or subtract it from QNH to get QFE.
For example, the airfield elevation is 200 feet.  Dividing by 30 gives us 6.66r.  The QFE is 1023. Add 6.66 to get 1029.66 and round up to 1030 millibars, which is the QNH.

In imperial:

1 inch mercury is approximately equal to 900 feet.  This calculation is therefore also approximate but is good for airfield elevations to several hundred feet since we round to the nearest hundredth inches.
Divide the airfield altitude in feet by 900 to get the number of inches above MSL. Add this to the QFE to get QNH or subtract it from QNH to get QFE.
For example, the airfield elevation is 300 feet.  Diving by 900 gives us 0.33r.  The QFE is 30.12. Add 0.33 to get 30.45 which is the QNH.
If the airfield elevation is below sea level, subtract rather than add and vice versa.

QNH and QFE are related by this equation:
$$\mathrm{QNH} = \mathrm{QFE}\left[1+\frac{hL}{T_0}\left(\frac{P_0}{\mathrm{QFE}}\right)^\frac{R_sL}{g}\right]^\frac{g}{R_sL}\tag{1}$$
where:

$h$: Elevation of the airport in meters
$L$: Temperature lapse $=0.0065 \mathrm{~K/m}$
$T_0$: Standard temperature $=288.15 \mathrm{~K}$
$P_0$: Standard pressure $=101325 \mathrm{~Pa}$
$g$: Gravitational acceleration $\approx 9.81 \mathrm{~m/s}^2$
$R_s$: specific gas constant for dry air $\approx 287.058 \mathrm{~J \cdot kg^{−1}K^{−1}}$

Check this post if you're interested in how equation 1 is derived (remember that QFE is the pressure measured at the airport).
It's easy to show how equation 1 is simplified into what Simon talks about in his answer. Using the Binomial approximation:
$$\mathrm{QNH}
\approx \mathrm{QFE} \left[ 1+\frac{gh}{R_sT_0} \left(\frac{P_0}{\mathrm{QFE}}\right)^\frac{R_sL}{g} \right]
= \mathrm{QFE} +\frac{gh}{R_sT_0} \mathrm{QFE} \left(\frac{P_0}{\mathrm{QFE}}\right)^\frac{R_sL}{g} \tag{2}$$
The above equation can be further simplified if we assume $\mathrm{QFE}$ never differs too much from $P_0$ (that is, $\mathrm{QFE} \approx P_0$, thus $\frac{P_0}{\mathrm{QFE}} \approx 1$). Hence:
$$\mathrm {QNH} \approx \mathrm{QFE} + \frac{gP_0}{R_sT_0}*h \tag{3}$$
And the constant $\frac{gP_0}{R_sT_0}$ is:
$$ \frac{ 9.81 \mathrm{~\frac{m}{s^2}} \cdot 101325 \mathrm{~Pa} }{ 287.058 \mathrm{~\frac{J}{kg \cdot K}} \cdot 288.15 \mathrm{~K} }
=
\frac{993998.25}{82715.7627} \mathrm{ \frac{m \cdot Pa}{s^2 \left( \frac{1}{kg} \right) \left( \frac{kg \cdot m^2}{s^2} \right)} }
=
12.017 \mathrm{ \frac{Pa}{m} }
$$
or in other words

roughly 0.12 millibars for every meter of elevation
0.97 inHg for every 900ft of elevation
1.0988 mb for every 30ft of elevation

which are the values that Simon talks about in his reply.

How accurate is this approximation?
Remember that we are applying some correction factor to QFE (a measured value) in order to estimate QNH. That is, we set the altimeter to equal 0 AGL; then we depart the local airport circuit and we want to properly set QNH on the altimeter to find our AMSL altitude without having to constantly add the displayed altitude to the known field elevation. We are performing some function, which has the format
$$ \mathrm{QNH} \approx \mathrm{QFE} + Ch \tag{4} $$
where $h$ is the AMSL elevation of the field where we set QFE and $C$ is a multiplier we apply to that field elevation. We have found three candidates for $C$ (based on the various approximation we applied to the original equation), namely:
$$ \begin{align}
C_1 &= \frac{gP_0}{R_sT_0} \tag{5.1} \\
C_2 &= \frac{g}{R_sT_0} \mathrm{QFE} \left( \frac{P_0}{\mathrm{QFE}} \right) ^ { \frac{R_sL}{g} } \tag{5.2} \\
C_3 &= \frac{g}{R_sT_0} \mathrm{QFE} \tag{5.3}  \\
\end{align}
$$
In order to assess which ones is the most accurate we need to see how much equation 4 differs from equation 1 for the 3 values of $C_1$, $C_2$ and $C_3$.
Equation 1 has 2 independent variables (QFE and h) but in practice, once the elevation is given, QFE cannot have any arbitrary values (and viceversa). It's reasonable to consider only values within the range $P_{\mathrm{ISA}} -4\% < \mathrm{QFE} < P_{\mathrm{ISA}} +4\%$, where $P_{\mathrm{ISA}}$ is the pressure one would find at the corresponding elevation in the standard atmosphere.
With this in mind, let's see how well the 3 multipliers $C_1$, $C_2$ and $C_3$ approximate equation 1. The following table reports the exact QNH for a set of altitudes and QFEs and the errors in percentage for the three multipliers ($C_1$, $C_2$ and $C_3$ respectively).

AMSL(m) \ QFE
P_ISA -4.0%
P_ISA -2.0%
P_ISA @AMSL
P_ISA +2.0%
P_ISA +4.0%

0
QNH 972.72 (+0.0%) (+0.0%) (+0.0%)
QNH 992.99 (+0.0%) (+0.0%) (+0.0%)
QNH 1013.25 (+0.0%) (+0.0%) (+0.0%)
QNH 1033.52 (+0.0%) (+0.0%) (+0.0%)
QNH 1053.78 (+0.0%) (+0.0%) (+0.0%)

500
QNH 973.17 (+0.34%) (-0.14%) (-0.25%)
QNH 993.21 (+0.24%) (-0.14%) (-0.23%)
QNH 1013.25 (+0.14%) (-0.14%) (-0.2%)
QNH 1033.28 (+0.05%) (-0.14%) (-0.18%)
QNH 1053.32 (-0.04%) (-0.14%) (-0.16%)

1000
QNH 973.62 (+0.96%) (-0.55%) (-0.88%)
QNH 993.44 (+0.75%) (-0.55%) (-0.83%)
QNH 1013.25 (+0.56%) (-0.54%) (-0.78%)
QNH 1033.05 (+0.37%) (-0.54%) (-0.74%)
QNH 1052.85 (+0.19%) (-0.53%) (-0.7%)

1500
QNH 974.07 (+1.84%) (-1.21%) (-1.85%)
QNH 993.67 (+1.53%) (-1.2%) (-1.78%)
QNH 1013.25 (+1.24%) (-1.19%) (-1.71%)
QNH 1032.82 (+0.95%) (-1.18%) (-1.64%)
QNH 1052.39 (+0.68%) (-1.17%) (-1.58%)

2000
QNH 974.52 (+2.97%) (-2.09%) (-3.12%)
QNH 993.89 (+2.56%) (-2.08%) (-3.03%)
QNH 1013.25 (+2.17%) (-2.06%) (-2.94%)
QNH 1032.59 (+1.79%) (-2.05%) (-2.86%)
QNH 1051.92 (+1.44%) (-2.04%) (-2.77%)

2500
QNH 974.97 (+4.34%) (-3.19%) (-4.67%)
QNH 994.12 (+3.83%) (-3.17%) (-4.56%)
QNH 1013.25 (+3.35%) (-3.14%) (-4.45%)
QNH 1032.36 (+2.88%) (-3.12%) (-4.34%)
QNH 1051.46 (+2.43%) (-3.1%) (-4.24%)

3000
QNH 975.42 (+5.95%) (-4.47%) (-6.46%)
QNH 994.35 (+5.34%) (-4.44%) (-6.33%)
QNH 1013.25 (+4.76%) (-4.41%) (-6.2%)
QNH 1032.13 (+4.2%) (-4.39%) (-6.08%)
QNH 1051.0 (+3.67%) (-4.36%) (-5.95%)

3500
QNH 975.87 (+7.78%) (-5.94%) (-8.46%)
QNH 994.57 (+7.08%) (-5.9%) (-8.31%)
QNH 1013.25 (+6.4%) (-5.86%) (-8.17%)
QNH 1031.9 (+5.76%) (-5.82%) (-8.02%)
QNH 1050.53 (+5.13%) (-5.79%) (-7.88%)

4000
QNH 976.32 (+9.83%) (-7.55%) (-10.65%)
QNH 994.8 (+9.03%) (-7.51%) (-10.48%)
QNH 1013.25 (+8.26%) (-7.46%) (-10.32%)
QNH 1031.67 (+7.53%) (-7.41%) (-10.16%)
QNH 1050.07 (+6.82%) (-7.37%) (-10.0%)

4500
QNH 976.77 (+12.09%) (-9.32%) (-13.0%)
QNH 995.03 (+11.19%) (-9.26%) (-12.82%)
QNH 1013.25 (+10.33%) (-9.2%) (-12.64%)
QNH 1031.44 (+9.51%) (-9.15%) (-12.46%)
QNH 1049.61 (+8.71%) (-9.09%) (-12.29%)

5000
QNH 977.22 (+14.54%) (-11.2%) (-15.48%)
QNH 995.25 (+13.55%) (-11.14%) (-15.28%)
QNH 1013.25 (+12.6%) (-11.07%) (-15.09%)
QNH 1031.21 (+11.69%) (-11.01%) (-14.9%)
QNH 1049.14 (+10.81%) (-10.94%) (-14.71%)

Observations:

$C_3$ performs the worst in almost all the cases, so it can be ignored
$C_1$ and $C_2$ perform pretty much the same and reasonably well (<= 2%) until 2000m
$C_1$ approximates QNH in excess, $C_2$ in defect (so $C_1$ is actually safer than $C_2$)
$C_1$ performs better with high pressure, $C_2$ is more accurate with low pressure

Conclusions:

The usual approximation of 1inHg/900ft or 1mb/30ft (i.e. $C_1$) is acceptable for practical purposes if the elevation is 2000m or lower.
Since it approximates the QNH in excess, $C_1$ is safer than the other 2 approximations evaluated (i.e. the pilot can fly to an altitude of 0 AGL without crashing on the ground)
In general, when possible, QNH should be taken from the METAR report (which likely uses equation 1 to compute it)

I don't quite follow how we got from eq 2 to eq 3. I would have said: (P_0/QFE)≈1 which eliminates (P_0/QFE)^(exp) and now we are left with QFE plus QFE times (gh/R_sT_0). But it seems you canceled out QFE/QFE (ignoring the exponent!!) and kept P_0. Is that how the math works?

...oh, of course, we are left with QFE times (const). But instead we write P_0 times (const) because we were assuming QFE≈P_0. Right.

@randomhead: Yes you got it right. And thanks for cleaning up the latex!

See my addition, especially the part hidden with an HTML comment block where I tried to quantify the error in calculating QNH with the different approximations (https://i.sstatic.net/HJVTp.gif). I got errors of greater than 2.5 inHg at high-but-not-crazy elevations (1100m/3600ft) so I feel like I must be doing something wrong. Or is the approximation just that terrible? Here's what I'm doing: https://i.sstatic.net/PTgq1.png

Let us continue this discussion in chat.

I just read about QNH and QFE so I thought I’d throw this out there to start the discussion. We don’t use QFE in the US but I think my logic is correct.
From the FAA Instrument Procedures Handbook
Barometric Pressure for Local Altimeter Setting (QNH)
A local altimeter setting equivalent to the barometric pressure measured at an airport altimeter datum and corrected to sea level pressure. At the airport altimeter datum, an altimeter set to QNH indicates airport elevation above mean sea level (MSL). Altimeters are set to QNH while operating at and below the transition altitude and below the transition level.
For flights in the vicinity of airports, express the vertical position of aircraft in terms of QNH or QFE at or below the transition altitude and in terms of QNE at or above the transition level. While passing through the transition layer, express vertical position in terms of FLs when ascending and in terms of altitudes when descending.
When an aircraft that receives a clearance as number one to land completes its approach using QFE, express the vertical position of the aircraft in terms of height above the airport elevation during that portion of its flight for which you may use QFE.
note that transition level is when you set the altimeter to a standard value. It varies by country. In the US it is 18,000' and 29.92 inches. Not relevant to this discussion.
In the US we don’t use QFE, but as a comment indicates, you would probably get that from the ATIS. If no ATIS is available, to convert QNH to QFE you would move the altimeter so that you decrease your altitude by the field elevation. Here’s an example:
My field elevation is 212'. If I set the altimeter to QNH it will show 212' as the altitude. To get QFE I need to change the altitude to 0'. In other words whatever altitude is showing on the altimeter, move the knob to make it show 212' less.
Note that when you turn the altimeter knob the altitude goes in the same direction as the pressure setting in the Kollsman window. In the US we use inches of mercury but the movement is the same: As an example, right now the altimeter setting at KSBP is 30.08. The setting at KSBA (62 nm away) is 30.01. If you fly to KSBA you would notice a change in altitude of -70 feet when you get the new altimeter setting from approach control.

"start the discussion" and "I think" are not good phrases to use in a Q&A site.  Otherwise, good answer.  You might consider an edit?

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Is NOAA GFS winds aloft data available for applications?
Where can I find the NOAA's winds aloft data in a form that I can use in an application? I plan to develop a software able to calculate the flight path of a lighter than air aircraft or drone using windspeed data from the NOAA Global Forecast System.

If you don't get an answer here, you might also try earth.SE

This X-Plane plugin pulls NOAA data (which I believe includes the GFS winds aloft). It uses a program called wgrib2 to parse the information. The plugin is open source, that should get you started.

See the following web page:
http://www.nws.noaa.gov/tg/general.php
This page describes getting started with the HTTP or FTP access to NOAA weather service APIs.

The GFS products can be accessed at http://www.nco.ncep.noaa.gov/pmb/products/gfs/.  You can use the filename scheme they list to pull down current forecast products from FTP.  These are the native output grids in grib2 format.

I'll update this with OpenDAP sources if I find any when I get a chance later.

It looks like the OpenDAP access might not be realtime, but you can take a look at http://nomads.ncdc.noaa.gov/dods/NCEP_GFS/ and once you drill down to a specific file you can see the URL model for that access method.

There are quite a lot of data sets on that page. Which one is winds aloft?

@TylerDurden the one labelled "GFS - World Area Forecast System
Upper Air Forecast (1.25 degree resolution)" has winds from 850 mb up to 100 mb.  Also the "GFS - Binary Universal Form for the Representation of meteorological data (BUFR)
Inital BUFR Data Dump Files" contain winds aloft / upper air data that is suitable for use in the BUFRLIB program.   I haven't looked, but the data behind http://ensemble.ucar.edu/sounding.php (WRF ARW instead of GFS) might also be available somehow.


text:
Must an instrument approach plate be visible during the approach?
Is there any legal requirement to display the approach plate for an IFR approach? Typically it's briefed as part of the descent checks and then kept on display - but I'm struggling to find any legal requirement to keep it visible.

"Legal requirement" where? Your profile says United Kingdom; are you interested in UK or EASA rules?

Why would there be one @andy-m? You cover the plate with a map and you're in breach.

Looking at EASA rules and regs.
@GdD - I'm not sure I follow, in breach of what?

If there is a regulation to display the approach plate and you cover it up you'd be in breach of the regulation @andy-m

If you can't find any language for that sort of thing there probably isn't any.  At some point the value of regulations ends and you have to let common sense and good airmanship prevail.

Under US 14 CFR 91, I am aware of no requirement for the plates to be on display.  The regulations do provide that the chart should be available, however.
One might ask themselves, prior to putting the charts in the case behind the pilot's seat...what would the accident review board have to say about this?


text:
Are most/all flights affected by a noticeable amount of turbulence?
How common is turbulence that is severe enough to feel uncomfortable? I don't mean a little shaking here and there, but a drop that lasts a second or two.
Last 2 flights I was on (Paris to Singapore, and Singapore to Sydney) had more than one big drop of turbulence, to the point where passengers screamed.
Is this common? I know it is relatively safe, but feel uneasy about flying again as it was uncomfortable and scary!

Possible duplicate of How dangerous was the turbulence on this commercial flight?

@Simon, I am not sure this is a duplicate. That question is about how to rate severity and how dangerous it is, this one is how common it is.

I'm not worried about the dangerous. I want to know how common it is

Well in that case, the question is asking for opinions. severe enough to feel uncomfortable.  I don't mind turbulence at all and I've been in some very rough stuff and often experience drops that "last a second or two".  I simply don't feel uncomfortable in so you need an empirical measure to ask how common it is.  E.g.  I can't remember being on a flight that didn't have some turbulence.  Maybe someone on the flight felt uncomfortable whilst I just enjoyed the ride.  Perhaps if you changed the question to ask "how common is moderate turbulence", since moderate has a defined meaning.

It's severe and frequent enough, and planned such certain to become worse with climate change, to see investments in research for better prediction and detection. These two studies were cited in this article in the Guardian. To get a specific figure, you probably need to restrict your question to a type of route (land or ocean) and a geographical area.

It's very difficult to answer this question as it depends upon so many different things:
 - Location
 - Time of day
 - Season
 - Flight level
 - Terrain below
 - Current weather (of course)
 - Type of aircraft
 - The crew and their knowledge / weather radar use.
What also makes it hard is that what 'feels uncomfortable' for one person might not for another.
In the tropics it's quite common for flights to be affected by turbulence. Most will see at least some light chop. In my experience of a few hundred long-haul flights through the tropics I've only had a couple with no moderate turbulence (what you might call uncomfortable). In a similar number of short-haul flights in Europe I'd say that around 50% had no significant turbulence at all and only around 5% had moderate turbulence.
You're more likely to experience turbulence in the tropics as there's a lot more convective activity there (air rising and falling). Away from the tropics the USA tends to generate turbulence whilst Europe does not (relatively speaking). SE Asia is particularly well known for turbulence encounters.

This is a fantastic answer, thank you

Clear air turbulence (CAT) is associated with jet streams. Depending on the region, and even the season, CAT is common, and it can't be identified by weather radar, but the crew usually studies the weather chart before the flight, and can receive pieces of information from ground units, ATCOs and for aircraft leading them.
So, most of the times, they can be aware of CAT, but it can also disrupt the flights, as you could feel during your journeys.

how common is 'common'? Every flight? Every 1 in 5 flights? And is CAT always severe (in terms of making people vomit and scream)

CAT is rarely severe. In the vast (>90%) majority of cases it's minor. There's only a handful of cases each year (out of all flights worldwide) where CAT is severe enough to cause vomiting, people being thrown around, etc). In my experience most "severe CAT" cases aren't really CAT at all, but a crew not noticing nearby weather.


text:
What is the expiration period for a 3rd class medical certificate?
I got my 3rd Class Medical for a Private Pilot certificate in May 2013 at the age of 39.  I'm now 41.  Does the 60 month rule apply to my certificate or do I need to retest under the 24 month rule for over 40 year olds?

According to the table in 14 CFR 61.23 (d), the expiration date of your medical is based on your age "on the date of examination for your most recent medical certificate."
So, if you were 39 on the date of the examination, you're good for 60 months.

There is a possibility that the requirement to renew 3rd class medicals may go away completely soon. On Dec 15, 2015, the U.S. Senate passed the "Pilot's Bill of Rights 2". The bill would still need to be passed by the House and signed by the President before it would take effect. As of Dec 15, it had 152 co-sponsors of both parties in the House. According to AOPA:

Under the medical reforms of Pilot's Bill of Rights 2, most pilots who have held a valid third class medical, either regular or special issuance, within 10 years of the legislation’s enactment would never need to get another FAA medical exam. The rule would apply to pilots flying VFR or IFR in aircraft weighing up to 6,000 pounds and carrying up to five passengers at altitudes below 18,000 feet and speeds up to 250 knots.
Pilots who develop certain medical conditions, including a small list of specific cardiac, mental health, or neurological conditions, will have to get an FAA special issuance medical one time only, significantly reducing the time and money spent navigating the FAA’s medical bureaucracy.
For pilots who have not had a valid medical in the past 10 years and those who have never applied for and received a medical certificate, a one-time third class medical certification by an aviation medical examiner will be required. After a pilot has been medically certified once, either through the regular or special-issuance processes, he or she will also be able to fly indefinitely without needing to go through the FAA medical certification process again.

Which is why private pilots should get a medical mere months before their 40th birthday. :)


text:
Can I work on my own airplane without formal mechanic's training?
Suppose I am not planning to be a career A&P mechanics, but would like to get some hands-on skills for airplane maintenance, for reasons of both cost and hobby. Say my basic goal is to perform tasks from as simple as tire replacement, instrument replacement to avionics installation and troubleshooting. My goals may be progressive. The aircraft for maintenance is self-owned or owned by share. The ownership and operation of the aircraft is within US.
Can I work on my own airplane? Since I'm not certified for mechanics work would it be OK that a certified A&P endorse my work is standard-compliant?

FAR Part 43 contains the information of what you can and cannot do to an aircraft and what requirements you must have.
If you are a certificated Private Pilot (as in not a sport pilot), there are quite a few things you can do to an aircraft you own, that is not used for commercial purposes. Like:

Replace wheel bearings
Remove/repair/reinstall wheels
Replace safety wiring and cotter keys
Service landing gear struts by adding oil/air
Lubrication not requiring disassembly of non-structural items like cover plates, cowlings, and fairings
Simple fabric patches not requiring rib stitching or removing structural parts or control surfaces
Replenishing hydraulic fluid
Refinishing decorative coating of fuselage and other parts
Repairing upholstery without disassembly
Simple repairs to fairings, etc
Replacing side windows
Replacing safety belts
Replacing seats or seat parts with approved replacements
Troubleshooting and repairing broken circuits in landing light wing circuits
Replacing bulbs, reflectors, and lenses of position and landing lights
Replacing wheels and skis where no weight and balance computation is required
Replacing cowling not requiring removing the propeller or disconnecting flight controls
Replacing or cleaning spark plugs and setting gap clearance
Replacing any hose connection except hydraulic connections
Replacing prefabricated fuel lines
Cleaning or replacing fuel and oil strainers or filter elements
Replacing and servicing batteries

And others Source
However, note that it has to be your airplane, and it can't be used commercially (which I think includes offering it for rent). There is very little you can do in the way of avionics (read #31 and #32 in the reference I gave).
The fun thing to note is that although a PPL is required, the PPL training does not give any special attention to any of these items, and therefore really somebody off the street is technically just as qualified as a private pilot, but the private pilot is the only one authorized to do the work (provided they own the plane). When I got my PPL, nobody walked me through replacing cotter pins or anti-back-out devices...
Edit
These rules though may be different when you get into experimental category aircraft but I can't source it now and will do more research and edit when I can clarify.

So this precludes a non-PPL helping out on a friend's non-commercial airplane, not for remuneration? i.e. If one is mechanically inclined can one "volunteer" on working on another's non-commercial airplane?

For avionics from your source, it looks have to be plug-and-play type, against rewiring. It would best I am as qualified as I am authorized in maintenance.

@curious_cat Helping and performing the work are probably two different things. If you have a buddy helping to hand parts to you or lift the tire up, I don't think that FAR 43 precludes them from helping, but its a grey area I think of how much needs to be done by the pilot. But no, I don't believe your car buddy can volunteer to change the wheel bearings on your 172 while you're across the street having a beer.

@spacepure Again, its difficult to interpret exactly, part of the wording in one of those says that "The approved unit must be designed to be readily and repeatedly removed and replaced, and pertinent instructions must be provided." So I'm guessing this is more like a dash mounted removable GPS unit, not a radio with connectors on the back.

You might want to add something about owner-assisted annuals, which are another common way that owners can do their own maintenance. An unqualified owner can't perform or sign off an annual, but they can help the mechanic who's doing it and of course it's a learning experience.

@curious_cat Maintenance can actually be performed by anyone, but in order to be legal, someone authorized to do the work must supervise, take responsibility for it, and sign it off as their own.

@Lnafziger, which tends to indicate that your car mechanic buddy could do the work so long as you, the owner with a PPL are there supervising, and sign off on the work. IANAP, IANAL, etc...

@freeman Absolutely, this is very clear in guidance from the FAA. Some "mechanics" at major service centers aren't even A&P's, but are doing work that is supervised by and signed off by one. This is a general rule that the FAA follows, including for parachute riggers, etc. They don't care who does the actual work as long as it is done correctly and they have someone governed by their rules who is responsible.

very typical of a trade with apprenticeships

Yes, the rules for experimental category are very different. Basically if you built the aircraft, you also get authorization to repair it, which is part of the reason kits are so popular.

This answer is misleading regarding ownership and commercial activity. Note that either an owner or an operator is authorized. The only commercial activity that disqualifies the aircraft is that conducted under part 121, 129, or 135.

But will anybody check, know, care if maintenance is performed professionally but by not certified person? :)

Please note, this answer is no longer correct, as the exhaustive list no longer exists. This was superseded by the Coleal Legal Interpretation. See https://aviation.stackexchange.com/a/87355/20394 for more info.

Short Answer:
Yes, you can work on your own plane, if:
I. You are performing maintenance work under proper supervision, or
II. You are performing preventive maintenance, and each of the following are true:

You are a certificated pilot (except sport pilot),
You either own or operate the aircraft in question, and
The aircraft is operated under part 91.

Long Answer:
14 CFR 43 regulates aircraft maintenance for those aircraft with a U.S. airworthiness certificate, but not for experimental aircraft (see §43.1 Applicability). For the purposes of this answer, I will use the term "aircraft maintenance work" to refer to any and all maintenance work on U.S. certificated aircraft that might fall under the legal definition of maintenance, preventive maintenance, and alterations. This does not include inspections.
§43.3 regulates who is authorized to perform aircraft maintenance work. Aircraft maintenance work is subdivided into maintenance, preventive maintenance, rebuilding, and alterations. For the purposes of this question, we are interested in two of the many description of who may perform aircraft maintenance work.
The first is found in §43.3 (d), emphasis mine:

(d) A person working under the supervision of a holder of a mechanic
  or repairman certificate may perform the maintenance, preventive
  maintenance, and alterations that his supervisor is authorized to
  perform, if the supervisor personally observes the work being done to
  the extent necessary to ensure that it is being done properly and if
  the supervisor is readily available, in person, for consultation.
  However, this paragraph does not authorize the performance of any
  inspection required by Part 91 or Part 125 of this chapter or any
  inspection performed after a major repair or alteration.

In other words, anyone can perform aircraft maintenance work under proper supervision. In your case, if you are working under an authorized mechanic who properly supervises you and signs off the work, yes, you can perform aircraft maintenance work.
The second is found in §43.3 (g), emphasis mine:

(g) Except for holders of a sport pilot certificate, the holder of a
  pilot certificate issued under part 61 may perform preventive
  maintenance on any aircraft owned or operated by that pilot which is
  not used under part 121, 129, or 135 of this chapter. The holder of a
  sport pilot certificate may perform preventive maintenance on an
  aircraft owned or operated by that pilot and issued a special
  airworthiness certificate in the light-sport category.

This is more complex. If we are to assume that you are the holder of a pilot certificate issued under part 61—for example a Private Pilot certificate—then you meet that part of the qualification. If you own your own aircraft, in whole or in shares, then you also meet that part of the qualification.
Now, you may also qualify if you only operate the aircraft regardless of ownership. 14 CFR 1.1 defines Operate as follows:

Operate, with respect to aircraft, means use, cause to use or
  authorize to use aircraft, for the purpose (except as provided in
  §91.13 of this chapter) of air navigation including the piloting of
  aircraft, with or without the right of legal control (as owner,
  lessee, or otherwise).

(Note that §91.13 defines careless or reckless operation. In other words, if you use an aircraft carelessly or recklessly you are not actually operating it according to the legal definition and so do not have preventative maintenance privileges for that aircraft. Now you know.)
So, if you own, pilot, or otherwise operate the aircraft, you may have preventive maintenance privileges. The final qualification will be whether you operate the aircraft under part 91—which would give you preventive maintenance privileges—or under parts 121, 129, or 135—which would generally deny you preventive maintenance privileges. Note that parts 121, 129, and 135 refer to scheduled or on-demand passenger or freight operations available to the general public, to put it simply.

Now, some of the answers here imply that you are not authorized to perform preventive maintenance if the aircraft you own or operate is used for commercial purposes.
This is not the case.
Many commercial purposes are allowed under part 91, including rental, flight instruction, aerial patrols, sightseeing, business transportation (think business jets), skydiving, and glider or banner towing, among others. If you otherwise meet the above qualifications and operate an aircraft for such purposes, you are legally authorized to perform preventive maintenance.

This leaves us with the question of what constitutes preventive maintenance. The legal list is found in 14 CFR 43 Appendix A (c) and other answers have done a good job of outlining that, so I won't go into it further.

Great answer, but please note that the Coleal Interpretation reframes the list of items in 14 CFR 43 Appendix A (to be exemplary and not exhaustive. C.f. https://aviation.stackexchange.com/a/87355/20394

Keep in mind you can work on your plane you own it and bought. It's yours, you can take cylinders off, poor sand down them, do whatever you want, it's your plane. HOWEVER, it's not airworthy once you do so, unless you are a certified mechanic or the work you are doing complies with what is allowed under preventive maintenance.
Do whatever you want if you own it, just remember it's not legal or airworthy to fly after your done until an authorized person releases it back into service.

Your answer is incomplete and factually incorrect. Anyone can work on their own aircraft under the supervision of a certified mechanic. You doing your own work does not automatically make it airworthy, but without a mechanic's license you cannot return the aircraft to service.


text:
What are concrete examples for class G up to 14500?
It is said that there are some places where class G goes up to 14500 and then class E starts. However, I find they all overplayed with some special use airspace by looking at NorCal and SoCal Sectionals. Can anyone point me to some non-trivial 14500' Class G places? It is best to see some civilian airport within it.

Are you only looking for examples from the USA?

@J.Hougaard Yes, I am for answers in US in this question.

@falstro: In US, Class E may start from surface, 700, 1200, 14500, and sometimes a customized AGL.

Ah, you're right there's also the jagged blue line. Forgot about that one. There's some funky E space over Nevada. :)

@falstro "in the lower 50"? Usually, the "lower 48" refers to the contiguous 48 states, excluding AK & HI. Was that a typo, or is that an aviation reference that I'm totally unfamiliar with?

@FreeMan typo. sorry. Too many for my fingers and toes... ;)

@falstro - just checking. It could be that we'd annexed Canada and Mexico, and I just hadn't noticed... :)

@FreeMan UK maybe? ;)

Lots of G to 14.5k in Alaska and Canada. Not much left in CONUS due to the proliferation of radar here.

It can be difficult to find these areas on a VFR sectional (there are very few of them anyway), but on an (FAA NACO) IFR en route chart they're shaded brown, so they stand out very clearly from the background. Here are a few grouped together in South Dakota.
IFR chart:

VFR chart:

At the western end of those areas there's a public airport, Bison Municipal (6V5):

There's so few of them it's a wonder they even bother to define it as 14500

The highest peak in continental US is Mount Whitney at 14,505'. Class E starts at 14,500. Probably not a coincidence.

@TomMcW This has changed substantially in the last 10-15 years. When I started flying there were lots of areas that I was familiar with that were Class E starting at 14,500. Now all those areas are gone, and only a few examples remain in the lower 48. I expect to see the trend continue, with a chance that current definitions may indeed become obsolete and superseded.

@Pondlife All of your examples are now gone.

There are two left.  Big bend national park in Texas and New Mexico near Cato MOA

@Stephen, examples? Scanning the continental US on skyvector, I only see those two. The areas mentioned in other answers are no longer G-to-145. There are some areas of higher G charted along the Mexican and Canadian borders, and some beginning a little ways out to sea, but the two Tim mentioned are the only ones I can find actually in the US. I don't even see any left in Alaska anymore.

@randomhead What the …!?  There was a ton all over the Southwest and Alaska last time I looked, and I didn’t think it was that long ago. I stand corrected.

Here is another example jewett mesa. Inside this polygonal shape is class Golf extending up to 14500 MSL:

Looks like is has been chopped in half since the answer, making Jewett Mesa now 1200ft below class G

Search for Airport (C54) for to find the Smitty MOA on skyvector.com. https://skyvector.com/?ll=34.14658987217066,-108.2852325395162&chart=301&zoom=3

L41 is another example for the case where the class E begins from 14500 and there is a public airport on the surface in the region.

Look in the Big Bend Area of Texas. Large chunk of what your are looking for on the sectional chart in that area.


text:
What preparations should I have to fly from the US to Mexico?
I am holding a US private pilot certificate with instrument rating, and planning to fly a single-engine aircraft from US to Mexico.
Regarding legal, visa, airspace/ATC knowledge requirements, flight plan filing, what do I need to prepare and how may I find the materials (in English).

Much of this is covered here: http://aviation.stackexchange.com/q/306/69

More importantly, where are resources to find the charts and frequencies of service?

AOPA has a guide here. I think it should cover most of your points apart from visa/immigration, because that isn't really an aviation issue (unless perhaps you're commercial air crew using a flight crew visa). Don't forget to plan ahead for re-entering the US, there are a bunch of requirements for that too, including using eAPIS.
Some points from the guide are:

Documentation: you need a radio license for the pilot and the aircraft
Insurance: add Mexico to your insurance policy if it isn't already covered
Flight plan: you must be on an IFR or defense VFR flight plan to cross the ADIZ
Night VFR isn't allowed in Mexico (with a few exceptions for flights to the US)
It looks like the more paperwork you have with you, the better
FBO services are likely to be more limited than in the US, and fees are often higher

Also check out the Caribbean Sky Tours Pilots Association, they publish a pilots' guide to Mexico and Central America.


text:
What does the number in degrees indicate on an approach chart?
On the following ILS chart, there is a number 301°. Is the number 301° indicating a track angle, heading or something else?

Ask yourself what makes sense.  What could be published and always be useful to the pilot in all situations and would maintain the same relationship with the fixed points on the plate.

@Jim that is not a helpful comment. What might seem obvious to you could seem very perplexing to someone starting out. APP CRS only means "Approach Course" for those of us who are used to the jargon. And showing an approach angle on the bottom right glidepath has always been a confusing representation. Snappish responses might feel temporarily feel good, but they don't celebrate the learning and documentation process, which is what SE sites are about.

@KennSebesta - That wasn’t a snappish comment. I’m sorry you took it that way. It was a comment in the vein of the Socratic method intended to stimulate critical thinking.

An approach plate is a map, so the directional values presented are always magnetic track angles (except near the magnetic North Pole). A heading is where the airplane points when it's flying.  So, 301° is the track of the localizer course, in degrees magnetic. Makes sense, as it's the approach to Runway 30.
In the Arctic, in some areas the magnetic compass is usable and some areas it's not, so the value will include a "T" (301°T) if the track is a True Track, or an "M" (301°M) if it's a Magnetic Track.  If there is no T or M, assume the value is magnetic.

Are you sure it's not the required bank angle?

That is the final approach course on the localizer.  You will fly that magnetic course once you are established on the localizer.  Your heading used to track that final approach course will vary depending on the winds aloft.


text:
Do pitot tubes account for reduced pressure at altitude?
Do pitot tubes account for reduced pressure at altitude?
Does indicated airspeed reduce with altitude? If so, how much?
Is it different for small planes vs big commercial airliners?

Welcome to aviation.SE! I’m not completely sure what you’re asking, but this question might be helpful.

No, pitot tubes DO NOT account for reduced pressure at altitude. The indicated airspeed from the pitot tube is only equal to true airspeed at sea level on a standard day. (15C and 29.92” pressure)
As soon as you start to climb in altitude, the indicated airspeed reading will have an ERROR. The indicated airspeed from the pitot tube will be less than the actual true airspeed, and the higher you go, the more the error will be.
BUT, this error can be advantageous in regards to the stall speed of the aircraft. The stall speed may be 60 knots true airspeed at sea level and 100 knots true airspeed at altitude due to the less dense air. The pitot tube indicated stall speed at altitude will still be 60 knots “indicated” due to the density change.
So when it comes to stall speed, the pitot tube indicated airspeed does seem to “account for altitude”, but it is not intended to do so.
Indicated pitot tube airspeed decreases from true airspeed roughly 2% for every 1,000 feet increase in altitude. It is not really correct to say true airspeed increases with altitude. It is really the indicated airspeed that decreases with altitude.
A Cessna 172 aircraft will cruise at 113 knots at 2,000’ on 72% power, but 72% power at 10,000’ will yield 121 knots. So you could also say that true airspeed at a particular power setting increases with altitude because there is less drag at altitude.
Large aircraft and small aircraft both have the same error when it comes to indicated vs true airspeed.

" The indicated airspeed from the pitot tube is only accurate at sea level on a standard day" - I think the word "accurate" is misleading there.  Assuming the pitot equipment is functioning correctly, the IAS measurement will be perfectly accurate.  It simply won't be equal to True Airspeed (but there is no real reason to expect or require these to be the same).

I edited my answer. Thanks.

No, the simple tube does not. However instrumentation may map the tube output according to a conceptual surface of multiple dimensions.
This is simple air pressure, there is no hot wire involved or MAF metering or choke sets applied to simple tubes. No hot wire, but there are heated and regulated tubes, a heat differential may be taken into account to estimate the mass air flow or density of the air.
Modern instrumentation may take into account temperatures, altitude, and use a moving sum of readings to smooth shocks or anomalies.
Read the documentation for the parts concerned.


text:
Does the fuel selector switch of a Cessna 172 automatically turn 180° when pulling fuel shut off?
I noticed this behaviour in X-Plane 11 using the LR Cessna model and I'm wondering if this resembles the correct behaviour of a real aircraft.
Also it seems that in X-Plane after pulling the fuel shut off you can't move the selector anymore to a single storage tank, but only by 180° at a time.
Is that behaviour "realistic"? How are the selector switch and the shut off connected in real-world aircrafts?

They are not connected in real aircraft.  Both are manual, mechanical controls, and the only thing the have in common is that they both control valves related to fuel.  What you are seeing is definitely a software glitch.


text:
Why were taxiways marked with numbers (as opposed to letters) back in the day?
I learned through the USAir Flight 1493 and SkyWest Airlines Flight 5569 collision that many taxiways at KLAX were marked with numbers (e.g., Taxiway 45, Taxiway 33) instead of letters (e.g., Taxiway B [bravo]). Below is a snippet of the airport diagram effective at the time of the accident.

Why were numbers used back then, and why has it changed to letters exclusively?

I would bet good money that it's a) because people usually number things when they want to be able to refer to them b) because they found out it's easier to confuse taxiway 17 with runway 17, and less easy to confuse taxiway Bravo with runway 17

"why has it changed to letters exclusively?" not exclusively. In some airports there are taxiways designated with [letter][number], usually for "stub" taxiways. See https://www.skybrary.aero/sites/default/files/bookshelf/3172.pdf

This memorandum on taxi way nomenclature states

Numbers by themselves, and the letters "I" and "O" must not be used
because they could be mistaken for a runway number.

The letter "X"
must not be used because a sign with an "X" could be misconstrued as
indicating a closed taxiway or runway.

Not only are they lettered but I,O, and X are not permissible. Its also worth noting that numbers can be used in some cases

After all available single and double-same alphabet letters have been
utilized, taxiways should use two-character alphanumeric designations
such as “A1.” (See Figure 1a) The numeric character used should be a
single digit from 1 to 9. (See paragraph f for the use of twodigit
designators).

The system was changed to prevent confusion of runway numbers with taxiway numbers.


text:
What is the difference between a reverse and non-reverse bootstrap ACM?
I'm doing a project for my school about aviation and I ran into reverse bootstrap air cycle machines (ACMs).
What is the difference between reverse and non-reverse bootstrap ACMs? Also, which manufacturer uses reverse bootstrap ACMs and for what? I would love to find more info about it, but it almost seems to as it would some kind of highly classified technology.

What research have you done? It would help us understand what you already know and what questions you still have

A brief google search found this. https://patents.justia.com/patent/20200346763

A normal bootstrap ACM takes hot high pressure bleed air, takes out some of the heat while still at high pressure, compresses it some more to boost the pressure and temperature again, takes out more of the heat while the pressure is still elevated, then lets it expand to drop the pressure, and thus the temperature, to a much colder temperature than where it started. The bootstrap effect(from the saying about pulling yourself up by your bootstraps), comes from amplification effect you can achieve by exploiting the kinetic energy in the flow with the compressor and turbine to amplify the temperature changes.
A reverse bootstrap works the process backwards, as a way to get an amplified (bootstrap) cooling effect from ambient ram air instead of high temperature/high pressure bleed. These systems are used for cooling secondary components via heat exchangers by enhancing the effect of ram air, instead of providing large volumes of cold air for cooling a cabin directly.
The air is basically passed through the ACM backwards, from the ram air inlet directly to the turbine, leaving the turbine expanded to a lower temperature than it was at the ram air inlet. This chilled air leaving the turbine goes through a heat exchanger that cools whatever it was you wanted cooled off (like an electronics rack). The air picks up heat from the heat exchanger, cooling the electronics you wanted cooled, more efficiently than if it was just ram air alone.
The job of the compressor in this backwards flow cycle is only to create suction in the ducting upstream of it, that is, the ducting from the turbine discharge, to the heat exchanger, to the compressor inlet. The suction enhances the temperature drop you get from the ram air expanding and dropping its pressure through the turbine. The turbine itself extracts energy from the from the ram flow in the work it does to drive the compressor, further helping with the chilling effect.
So in the reverse bootstrap cycle, instead of amplifying hot high pressure bleed flow's pressure and temperature to get a bigger temperature drop than otherwise, and discharging expanded cold air for directly for air conditioning, instead you amplify the cooling effect of ram air that you pass directly through the expansion turbine first, and the compressor's job is just to generate suction, further dropping the pressure of the turbine discharge to cool it more. And the compressor's discharge just gets dumped overboard having done its job.
To summarize, the reverse bootstrap effect is the use of the kinetic energy in the ram air flow to drive the turbine to drive the compressor, which creates suction to amplify the temperature drop at the turbine.

"In a so called reverse bootstrap ACM ... RAM air may be first expanded to a sub-ambient pressure and cooled in the turbine and directed to a heat exchanger before being compressed back up to ambient pressure and expelled. The heat exchanger in this configuration is used to remove heat from another source, resulting in heating (rather than cooling) air flowing through the reverse bootstrap ACM."
This text is sourced from patent US11014677B2 for such a mechanism held by Hamilton Sundstrand Corp. It's the same patent found by John K above but Google Patents shows more complete information.


text:
Why do we calculate 25% more of TODR?
In general aviation aircraft, we are being told to calculate TODR (takeoff distance required) using AFM and TODR x 1.25. Why do we need the value with 25% added?

This is called a "safety factor", and I was actually taught to use 1.33!
When you look at the POH or AFM the numbers and charts they present for performance figures were invariably calculated using a near-perfect/brand new aircraft, with a highly skilled test pilot.
That club aircraft you're learning in may well be older than you are, the performance of the engine not quite what it was at zero hours - and let's be honest your skill on take off might not yet be up to the level of that test pilot.
So in almost every performance calculation you figure out you'll want to add some amount of leeway from the published numbers to account for the aircraft and/or your personal skill level - be that take off, landing, minimums on an instrument approach etc.

And don’t forget rejected takeoffs!


text:
What is the difference between A321neo and A321neo LR?
According to the illustration on this Airbus webpage about the A321Neo XLR, the A321Neo has 3500 NM range and A321Neo LR has 4000 NM range which is in conflict with this Airbus website about the A320Neo, the A320Neo has 3400 NM range, and in this Airbus website about the A321Neo, they claim the A321Neo to have a 4000 NM range.
I wanted to know whether there is a difference between A321Neo and A321Neo LR or are they referring to the A321Neo as A321Neo LR?

The website about the A320 has a different figure than the A321 because they are different airplanes.

The Airbus website is not incorrect, but there is a slight inconsistency between how the A321 LR is referred to. On one page it is marketed as a special aircraft, on the other it is implied that it is just one of the A321NEO varieties.
Note that the A320 page linked in your question is unrelated; it is a different aircraft.
The A321NEO and the A321LR are very similar aircraft.
In the certification data sheet, the A321NEO is designated as A321-2xxN (xx either 51, 52, 53 , 71 or 72, indicating engine model). These aircraft are build with either 3, 4 or 5 tanks. The basic aircraft has 3 tanks; two wing tanks and a centre tank. Optionally, one or two Additional Centre Tanks (ACT) can be installed.
A variety of the A321NEO is approved as A321-2xxNX (same xx numbers). This version can have an increased maximum ramp weight and maximum take-off weight (up to 97 metric tonnes) and offers up to three ACTs. The option with three ACTs is marketed as the A321 Long Range. In the certification, the A321LR is a variety (modification) of the A321NEO.


text:
What is the ILS glide slope deviation on a 4 degree VOR check error?
On a US IFR flight, it is required to do a VOR check every 30 days and the error tolerance is 4 degree except for airborne check.
Suppose I have 4 degree error on my VOR check. When I fly an ILS and right on the indicated glide slope, what is the maximum degree of deviation from the actual glide slope? (range of my actual glide slope)

Why would a deviation in a VOR check affect the ILS glideslope?

VOR/ILS uses the same VHF NAV system, so if there is an error on VOR frequency, it may imply an error on ILS frequency as well.

But the VOR system works on a 108.00-117.95 MHz, frequency & amplitude modulated signal. The glideslope works on a 329.15-335.0 MHz amplitude modulated signal. Both have different signal paths to the indicator instrument, and they use different needles on the indicator.

@skyoasis While VOR and ILS systems do often appear in the same box, the ILS system uses circuitry other than that which degrades in the VOR system and which necessitates the VOR accuracy check.

The deviation detected on your VOR check is likely to be unrelated to the deviation of the glideslope. Whilst they use the same indicator instrument, the radio systems driving the instrument are typically separate.
The VOR system works on a 108.00-117.95 MHz, frequency & amplitude modulated signal. The glideslope works on a 329.15-335.0 MHz amplitude modulated signal.
Both signals have different paths to the indicator instrument, and they use different needles on the indicator.
Therefor it is unlikely that a systematic error causing a deviation in the VOR omni-bearing indicator will cause a deviation in the glide path indicator.
There may be common error sources in both indications, but that depends really on the internal design of the receiver. For example if there is a voltage bias on some part of the system it may affect both systems. But you can't say anything about the maximum degree of deviation from the actual glide slope based on the observed deviation in the VOR receiver.
The localizer receiver works on the same frequencies as the VOR, but the demodulation is different. In an analogue system with needles, they use the same needle. So for localizer there is a higher system commonality with VOR than in the glide path case, which increase the likelihood of a common source of error.

So are you saying that the deviation error in VOR frequency has nothing to do with the error of its paired glide slope frequency on the same unit? The errors are independent, without correlation just like two separate units?

I am not saying they are completely independent, I am saying it is unlikely that a deviation in VOR will also show as a deviation in glideslope. There may be common error sources in both indications, but that depends really on the internal design of the receiver. For example if there is a voltage bias on some part of the system it may affect both systems. But you can't say anything about the maximum degree of deviation from the actual glide slope based on the observed deviation in the VOR receiver.

@DeltaLima Does the same hold true for the localizer needle, i.e., if NAV1 fails a VOR check, is it trustworthy for a LOC approach?

@GregBacon The ILS LOC works on the VOR frequencies, but the demodulation is different. In an analogue system with needles, they use the same needle. So for LOC there is a higher system commonality with VOR than in the GP case, which increase the likelihood of a common source of error.

The difference is that the vor receiver has to compare the phase difference between two 30Hz tones so requires the frequencies to be right on the number. The ILS compares the amplitudes of a 90Hz tone and a 150Hz tone. Since it doesn't need to calculate a phase relationship it doesn't require such accuracy. Also, the VOR accuracy is worse at long distances. 4 degrees is a big difference at 100 nm. The ILS is the same way, but you're not using it from 100nm and it gets more accurate as you get closer.

@DeltaLima Suppose we tune an ILS frequency that supports LOC, GS and DME. So basically, the error showing on one of the three indicators does not imply the errors of other two? (The other two may or may not have errors). On the other side, the integrity of any of LOC, GS, DME does not warrant the integrity of the other two? The three indications should be tested separately.

@skyoasis that's sums it up nicely.

@TomMcW, the two 30Hz tones a VOR receiver compares both come from the received signal and are both derived from the same oscillator, so they trivially match. Of course, the method of comparison is still very different — VOR needs to phase-shift one of the tones and add them to measure interference and the inductive element that does that (which is attached to the OBS knob) is not used for ILS.


text:
What runway alignment should I expect from a straight-in approach?
More precisely, in the case when a straight-in approach is present, what is the maximum angle between the final approach course and the runway heading for a straight-in minimum apply?
I had thought it is 15° for GPS IAP and 30° for others. However, when I looked at RNAV (GPS) RWY 28L @ KMRY, the angle is |279.4° - 261°| = 18.4° > 15°. Is there any other rule that applies, or is this simply an exception?
Note that on IAP runway diagram (bottom-left), there is usually an arrow with a distance from FAF to MAP and course direction, and they are omitted here.

According to the FAA Instrument Procedures Handbook, Chapter 4, p 4-63: For a non-vertically guided straight-in RNAV (GPS) approach, the final approach course must be aligned within 15° of the extended runway centerline. That doesn't conclusively answer the question, but does lead me to wonder if the LP approach is considered straight in here, but the LNAV approach is essentially considered a circling approach. But then why have the LNAV option at all?

Also I noticed that on G1000/GFC700 WAAS-equipped database, there is no procedure for LP approach, only LNAV.

This is an interesting question because it turns out that RNAV approach criteria aren't defined in the TERPS as you would expect. Instead, there's a separate FAA order 8260.58 for RNAV approaches.
The TERPS does indeed give 30° as the maximum offset for most approaches (e.g. section 4-2-4) and 15° for VOR/DME RNAV approaches (section 13-3-4), but VOR/DME RNAV approaches are still based on ground stations (section 13-1):

VOR/DME RNAV systems use signals based solely upon a VOR/DME, VORTAC,
  or TACAN reference facility. These systems use radials and distances
  from a reference facility to compute position and flight track
  information

On the other hand, the criteria for 'true' RNAV approaches not based on ground stations are defined in 8260.58 and they allow for offsets up to 30°  for approaches without vertical guidance (section 3-2-2):

a. Offset. When the final course must be offset, it may be offset up
  to 30 degrees (published separately from vertically guided) when the
  following conditions are met:
(1) Offset ≤ 5 degrees. Align the course through LTP.
(2) Offset > 5 degrees and 10 ≤ degrees. The course must
  cross the runway centerline extended at least 1500 feet prior to LTP
  (5200 feet maximum).
(3) Offset > 10 degrees and ≤ 20 degrees. The
  course must cross the runway centerline extended at least 3000 feet
  prior to LTP (5200 feet maximum). For offsets > 15 degrees, CAT C/D
  minimum published visibility 1 SM, minimum HAT of 300 feet.
(4) Offset 20 to 30 degrees (CAT A/B only). The course must cross the runway centerline extended at least 4500 feet prior to the LTP (5200 feet
  maximum).

Since the approach you showed doesn't have vertical guidance, it would fall under this section and therefore the offset of 16.76° marked on the plate is normal.


text:
Is it possible to perform a spin recovery in IMC?
In other words, after some training, is it possible to recovery a spin solely by relying on instruments?
If using a vacuum-pump traditional attitude indicator, I assume the answer is no, since there are limitations for its pitch and bank indication. What about an AHRS-based attitude indicator? Is it capable of matching 360° attitude changes in all directions, and catching up (without delay) with fast rolls? Can it filter out vibration well enough, so that a pilot is able to perform a spin recovery maneuver?

Given enough altitude, some aircraft will recover without pilot control inputs. Aircraft like the cirrus the recovery procedure is to pull the chute.A lot of being able to recover or not depends on skill and altitude.

I would think that all things are possible. As @RonBeyer hinted, how likely you are to recover from a spin in IMC would be determined by your altitude, skill, and ability to not freak out.

In general though I don't think its possible unless you break out of IMC with 5000+ feet of altitude to spare. You are going to have bad spatial disorientation and not be able to trust the instruments. The chances of recovering from a spin while still in IMC are statistically close to zero.

@RonBeyer The Mueller/Beggs spin recovery technique (assuming it works in your aircraft) will recover just fine in IMC.

@DanPichelman I'm not suggesting that normal spin recovery techniques wouldn't work in IMC, what I'm saying is that your spatial disorientation and brain may not allow it. That is why a Graveyard Spiral is such a deadly thing. You have no visual references so even recognizing that you are in a spin will be difficult.

I, for one, would like to do spin recover under the hood with a CFI

@rbp It's fun the 3rd and 4th times. But that's about it. I definitely think spin awareness in all modes and types of flight should be a much bigger focus.

I know everyone has his opinion on this subject and there should be spin awareness training, but personally, if I had an instrument instructor who decided that the way to prep me for an instrument checkride was to do simulated instrument spins, I would probably fire him for milking me for cash and not utilizing my flight time to the benefit of maximum checkride performance; IMHO.

@MortensenAviation. I didn't do it for the checkride. I did it so that if it ever happened, they wouldn't panic. And I only did it with my exceptional students that felt comfortable doing it. I followed the PTS in my teaching, but I've always believed there's a bit more to being a pilot than just what the test says it is. I can promise you that none of my students ever felt milked.

Why didn't you ask about the turn rate indicator or turn coordinator?  (Answer might not be the same for one as for the other.)

I do not want to speak for civilian training here, and I do know that each aircraft has its own set of emergency procedures. Those procedures will depend upon the instrumentation that you have available to you. In my case I had full IFR instrumentation with a state of the art inertial navigation system. Spin recovery should be in the set of immediate action emergency procedures that you have memorized, and you should be able to perform them in your sleep. Can you recover from a spin while in IMC? Most definitely, and I speak from experience.
I flew high performance tactical jets for the US Navy. This included the T2, the A4, and the A7E. We only did spin recovery training in the T2. It was a lot of fun. High performance jets are often flying on the edge of their performance envelope. Consider air combat maneuvering where you can easily find yourself in uncontrolled flight, departing the aircraft in a high-g turn, or even nose high at 0 airspeed. I was told it was quite difficult to spin the A7E, which is what I flew in the fleet.
In the Navy we were trained never to look out the cockpit if we thought we were in a spin. There was unusual attitude recovery training under the hood, as well as learning how to recover from departed flight (approach turn stall), and spins. It was drilled into us from day one to come in to your instruments for spin recovery. It would not have mattered to me whether I was IFR or VFR, the decision on recovery was strictly based on instruments. We were told if you look outside the cockpit during a spin you might misinterpret the visual cues, and while not actually in a spin, place the aircraft in one by applying spin recovery techniques. The procedures were something like:

Altitude: below 10,000 feet eject
Angle of attack: oscillating above 23 units of attack
Airspeed: oscillating between 110-140 knots
Turn needle: pegged
All four conditions met: apply spin recovery procedures

I flew over 30 years ago and I still remember these procedures. You might enjoy how I came to appreciate the seriousness of spin recovery. I had a decorated Skipper who had flown all through Vietnam in the A4 as an attack pilot. He stood up one day in the ready room and told us young'uns, "I don't care what you do out there, just make sure you brief it before you do it. Too many of us die because we impulsively decide to do something without thinking it through completely. Know your emergency procedures."
One night I was out at 25,000 feet tooling around after completing my mission, waiting to recover at the ship. A bit bored I had the thought, "I do loops in the simulator where it is blacker then this back home all the time. I have plenty of altitude to get the airspeed I need ... Why not?" And down I went. Unloading the aircraft to negative 1 or 2 G's I picked up airspeed well above what I needed and started pulling back up into the loop. It was dark out, no moon, with a solid cloud layer at 5,000 feet. I remember having an uncomfortable, anxious feeling at some point. Something wasn't right. I started paying attention. It was quiet. There wasn't the usual engine and wind noise! I came into the cockpit and the first thing that caught my eye was the full white of my rather large attitude indicator located in the middle of the instrument cluster. I had relaxed back pressure on the stick at some point. Oh my God. Not what you wanted to see in an A7: nose pointed straight up. Quickly glanced at my airspeed indicator to the left and watched it drop like a rock to the peg at 0 knots. Then the aircraft dropped backwards pushing the tail, and then entering a roll. I was along for the ride though. I wasn't thinking. I wasn't present. Spin recovery didn't enter my thoughts. Never checked my altitude. The aircraft dropped like a leaf for a bit and then settled in a descending roll. I put in opposite aileron and came out of it. Whew! Of course my gyros were toast and the inertial navigation system useless. I ended up coming through the solid overcast and doing my instrument approach to the ship on pressure instruments. I always followed my Skippers advice after that.
Eventually we came ashore and I found myself doing regular emergency procedure training in our simulators. Towards the end of one of these sessions I decided to practice spin recovery. I knew we couldn't get the simulator into a spin, but I thought it might be useful nonetheless. So every time I got to the simulator I made sure I practiced.
The next time I found myself at sea I had finished my flight, and was getting together with a section to practice air combat maneuvering: 1-v-2. We had agreed to meet at a point over the ocean, at a given time, with an altitude separation between flights. I had everything below 15,000 with 5,000 as the hard deck. they had 16,000 an above. You could get there early, or late depending on how you wanted to try to set up the fight. Eventually we set up radio comms. The rule was that we had to talk to one another until one of the flights had gained sight and could then maintain separation.
As I came in to the fight I decided to make a gamble. I unloaded the aircraft with 2 negative G's and descended quick to 5,000 feet. They wouldn't be looking for me that low and so I would be outside their scan for bogies. The top of my aircraft was painted this dull off gray and it blended in with the haze and blue-gray of the ocean underneath. The sky above was clear blue. The tactic payed off and I saw the flight at around 18,000 feet. They knew they were in trouble when I stopped answering their radio calls. I pulled up into an Immellman, but had bled off a lot of airspeed from the dive as I was level at 5,000 feet in the denser atmosphere. As I got to 18,000 feet I was directly behind the two airplanes and I saw my airspeed hit the peg. I quickly called 2 Aim-9 missile shots to win the fight. Then on my back at zero airspeed the automatic flight control kicked the rudder inadvertently, and the A7 entered a spin. How do I know? Well, the AOA was above 23, the airspeed was between 110-140, and the turn indicator was pegged. It was second nature and I was in full control of the situation. As I was assessing the situation I got a call from the other flight lead asking, "Hey where are you?" I replied calmly, "I am a bit busy hold on ..." After recovering from the spin we headed home.

Great story!  Thanks for sharing it!

Entirely possible, although it is the turn coordinator not the artificial horizon that is primary -- to determine the direction of the spin.  Nose-down elevator, rudder opposite the spin, the when spinning stops recover from the dive using airspeed & altimeter for pitch control & the turn coordinator for roll. Those instruments don't tumble.
I'm told that in the 1950s, spin recovery in simulated IMC was a requirement for some ratings (commercial, instrument, and/or ATP - not sure which).  Not so much now, obviously, but with a suitable airplane and appropriate instruments, it is entirely possible.

When I used to teach my IFR students, they usually spent only the first couple of hours with all of their instruments. I put them in all sorts of unusual attitudes and let them recover under the hood, and with "failed" instruments. And I sometimes included spins with the students I thought could handle it. That was what my instructor did to me, and I guess I carried on the tradition. It sucked. And I know my students had horrible thoughts of me at the time. But it made us better pilots. Especially in IMC. It taught me not to rely on the things that were  more likely to fail and to quickly be able to interpret what my instruments were telling me. I went for a month in training thinking the Attitude Indicator and Directional Gyro looked like a suction-cupped soap holder. I don't think it's hard to tell you're in a spin; you just have to know what your plane is saying to you.
To your question: Is it possible to recover? Absolutely. Is it likely though? It depends. If you have a lot of altitude and you're in an aircraft that has decent spin characteristics, probably (especially since you mentioned prior spin training). But if you were at a couple thousand feet in a Piper Tomahawk, probably not. Would I recommend trying it? A big resounding NO!!!
And to your other question about AHRS: In a spin, the Attitude Indicator, no matter how accurate it might be, was the last thing I looked at in recovery. Unless it can demonstrate 100% reliability in all flight attitudes, it's safest to just assume it's lying to you.
Side Note: I recently did a balance function test with a physical therapist for an inner ear vertigo issue. She told me that pilots were some of the hardest people to test, specifically because we teach ourselves to ignore what our ears tell us. :-)

I'd imagine you're like that really hard nosed high school coach - everyone hates you during practice, but, years later, they look back and realize you were their favorite.

:-) Most of my former students went on to airline careers. I hope they remember the lessons I taught them, and I hope they feel like I do about my hard teachers.

Yes it is possible, but it takes more than training; it takes practice
Point of reference/experience: Flight Instructor and Standardization Instructor, three years, US Navy.

Caveat 1: I'd suggest not getting into a spin in IMC in the first place, by flying smoothly and making "do not stall" an imperative that your life depends on.  (See the AF 447 accident, an A330, which was not even a spin -- "merely" a stall -- that killed them all.  The investigation showed that spin in IMC and stall in IMC training, was not part of the A330 training syllabus.  Approach to stall/stall prevention was).
If you want to be prepared for the unusual case, you still have to both train for it AND *stay proficient.  Some skills atrophy with disuse but can be relied upon when exercised -- just like a muscle.  Instrument scans and partial panel instrument scans are in that skill category -- big time.
Caveat 2: Make sure you are already proficient at spin recovery in
VMC/VFR in your aircraft
In the following discussion, the term Attitude Gyro means your aircraft's primary attitude reference system, which is your primary scan item when flying in instrument conditions.

Two cases:

With Attitude Gyros that don't tumble (Can handle the 3-d environment).
With Attitude Gyros that tumble (can't handle the 3-D environment)

Case 1
You are able to use the attitude gyro and your other instruments, so you can adapt whatever full panel instrument scan appropriate for that maneuver (spin recovery) just as you could do aerobatics like a barrel roll or half Cuban eight on the gauges.  (The FTI for the T-45, barrel roll, is on page 3-24 of this link).  Whatever the case, follow the procedure for your aircraft and keep that instrument scan going.  Do everything in your power to ignore the weird feelings that the 3-d rotation is telling your body, and trust your instruments.
WARNING:  Use the procedure specific to your aircraft, the procedure for spin recovery.  In general, this means that you:

confirm spin (which is a stalled condition with rotation);
confirm direction of spin;
apply anti-spin control inputs specific to your aircraft;
stop spin (and note that airspeed is increasing);
If you don't have an Angle of Attack indicator, your airspeed and attitude are your cues
recover from ensuing unusual (usually nose down) attitude.

Case 2
In case 2, you are dealing with a serious emergency procedure, since you have to rely on a partial panel scan as your attitude gyro will tumble. (That means that it will lie to you).   This is from personal experience.  I had to do IMC spin recoveries to earn my special instrument card -- I also ran a number of Instructors through this drill for their training.
You have to do everything for Case 1 but you have to do it While Ignoring That Non-Functioning Attitude Gyro, and forcing yourself to smoothly stop the (disorienting) rotation by only scanning your other instruments: Airspeed/Turn Needle-Ball/Altitude/VSI ... all of which are cross check instruments in a normal instrument scan based on the attitude gyro.  This can be difficult to do since your trained reflex, your habit, is to refer to your Attitude Gyro in IMC.  It takes an act of will to ignore it and only scan the other instruments.
The First Time You Try it you may not get it right
During my first ever "spin under the bag" the Standardization Instructor (SI) had to take the controls since I had un-stalled/unspun the aircraft but I was doing a 'rolling pull out' which our NATOPS manual prohibited.  My next one was acceptable, if not pretty.  I had been teaching instrument flying, also teaching partial panel techniques, and flying both for about a year.  I was both current and proficient in both and flying 30-40 training sorties per month, about a third of which were instrument sorties.  Even if you are a decent instrument pilot, it can be a challenge.
When I was the SI, I noticed that most of the pilots took one or two tries to get it right and then would have the technique down.
Why proficiency matters
I caught mononucleosis and was grounded for two months.  My back-in-the-saddle flights (I got 3) was to get me back up to speed on all quals, and that included "spin under the bag" since I was a Stan Instructor.   My first attempt was barely satisfactory (my completed recovery was ~ 400 feet above the hard altitude, and my SI debriefed me that he nearly took the controls).   I'd been doing and teaching this for six months, I knew the procedure, but being a little rusty was all it took to nearly goon it up.  I can still recall how hard I worked on that recovery to keep my scan going and be smooth.  It was humbling.
Bottom Line
Know your aircraft, know your spin recovery, practice your instrument scan, and (if you have a gyro that tumbles in a spin) practice your partial panel scan.
It can be done, but you have to keep those (procedures) muscles toned.  Spin in IMC (and for that matter, stall in IMC) is a case where An Ounce of Prevention is far better than a Ton of Cure: don't get into a spin (or even a stall) in IMC.  Make that a priority like your life depends on it.  (It does).

A pleasure to serve.  Fly safe! :-)

It's certainly possible, but also highly unlikely. It's possible because - ignoring thunderstorms, icing or other extreme conditions - being inside a cloud doesn't change how the aircraft handles or how to recover from a spin.
The real challenge would be to recognize that you're in a spin and react quickly. With no visual reference, the only way to know you're spinning is to deduce it from the instruments, and it could be very difficult for you to interpret them correctly and in time to prevent a crash. Even more difficult if some have toppled or become unreliable.
It's true that "unusual attitude" recovery is part of instrument training in the US (and probably most other places), but the training exercises are usually based on steep, turning climbs and descents or near stalls. I couldn't find any solid information on spin training under the hood, but personally I think it's highly unlikely that most pilots would survive, unless perhaps they have an airframe parachute and deploy it promptly. Or they break out of cloud with enough altitude - and presence of mind - to recover safely. I've never heard of an instructor spinning an aircraft with a student under the hood, although I suppose there's no specific reason not to.
Finally, it's often mentioned that in the early days of aviation, spinning was used as a way to descend through a narrow hole in clouds or even directly through the clouds. It was a 'convenient' way to lose altitude quickly while staying in more or less the same place. On the other hand, I've never read a first-person account of that (maybe someone else has?) and without knowing for sure where the cloud bases are it would be extremely dangerous. But every description I've seen also says that pilots spun through the clouds before recovering in VMC, which is a different situation anyway.

Having flown the A-4 Skyhawk series from 1958 to 1972 I concur with the A-7 jock. After having been at sea for 6-8 months all navy Air Wing pilots were scheduled for a week of ATC/instrument training. We always ended with stall/spins using partial panel-no gyro.